Optical cables include a core having one or more optical fibers within a sheath system, which surrounds and protects the fibers. In the cable industry, it is well known that changes in ambient conditions lead to differences in water vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level within the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. Water may also enter the cable because of rodent attacks or mechanical impacts that cause openings in the sheath system. And while the presence of water within an optical cable is not necessarily detrimental to its performance, passage of the water along the cable interior to connection points or terminals or associated equipment inside closures, for example, may cause problems especially in freezing environments and should be prevented. Therefore, it should be no surprise that cables for transmitting communications signals must meet industry standards with respect to waterblocking provisions. For example, one industry standard requires that there be no transmission of water under a pressure head of one meter in one hour through a one-meter length of cable.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and along the core. For example, metallic shields have been used that require a longitudinal seam to be watertight. However, forming a shield around the cable core requires relatively low manufacturing line speeds, and destroy the otherwise all-dielectric property of an optical cable. Moreover, lightning strikes are known to create holes in such a metallic shield, thereby damaging its integrity as a water barrier.
Various gel-like materials have also been used to fill cable cores to prevent the ingress of water with varying degrees of success. And while the use of such materials cause housekeeping problems for field personnel during splicing operations, they continue to be used in optical cable cores because they effectively block the entry of water and maintain the optical fibers in a low-stress state. These materials generally comprise a thickening or gelling agent in a liquid carrier. The gelling agents used are typically fatty acid soaps, but other agents, such as clays, silica, organic dyes, aromatic amides, and urea derivatives are also used. Non-soap thickeners are typically present as relatively isometric colloidal particles. Gelling agents form a network structure in which the liquid carrier is held by capillary forces. When a low stress is applied to a gel-like composition, the material acts substantially as a solid. If the stress is above a critical value, then the material flows and viscosity decreases rapidly. (Materials having such characteristics are called "thixotropic.") This decrease in viscosity is largely reversible because it is typically caused by the rupture of network junctions between the filler particles, and these junctions can reform following the release of the critical stress.
A cable filling material, especially an optical fiber cable filling material, should meet a variety of requirements. Among them is the requirement that the physical properties of the cable remain within acceptable limits over a wide temperature range, for example, from about -40.degree. C. to about 80.degree. C., and the filling material should not drip out of the cable at temperatures as high as 80.degree. C. It is also desirable that the filling material be substantially free of syneresis, which is to say that it should have uniform consistency over a predetermined temperature range. Generally, syneresis is controlled by assuring dispersion of an adequate amount of colloidal particles or other gelling agent. Another desirable property of the filling material is thermal oxidation resistance.
Filling materials for use in optical fiber cables should yield under strains that are experienced when the cable is made or handled. Otherwise, movement of the optical fibers within the cable would be prevented and the fibers would buckle because they contact a surface of the unyielding filling material. Filling materials should also have a relatively low shear modulus, G.sub.e. However, it has been determined that, at least for some applications, a low value of G.sub.e of the filling material is not sufficient to assure low cabling loss, and that a further parameter, the critical-yield stress, .sigma..sub.c, needs to be controlled because it also affects performance. It has been found that cabling loss is kept suitably low when .sigma..sub.c is less than about 0.01 psi (i.e., 70 Pa).
A filling material that exhibits relatively low critical-yield stress is disclosed in U.S. Pat. No. 4,701,016 that issued on Oct. 20, 1987. This filling material comprises oil, a gelling agent such as fumed silica particles and, optionally, a bleed inhibitor. Fumed silica particles are used as inorganic thickening agents to adjust the yield stress of the composition, and cause the oil to gel by bonding surface hydroxyl groups to form a network. Such gels are capable of supporting a load below a critical value of stress. Above this stress level, the network is disrupted, and the material assumes a liquid-like character and flows under stress.
Filling materials for use in cables also must pass industry-standard drip tests. To pass these tests, the filling materials must be retained as cable samples, suspended vertically, are subjected to specified elevated temperatures. Some prior art materials perform satisfactorily with respect to microbending and associated losses, but they bleed out excessively and have problems in meeting current drip tests. Also, it is desired that the low mean added losses exhibited by some prior art filling materials at least be met by filling materials which pass the drip test and have suitable low temperature properties.
Oil separation is a property of a filling material that describes its tendency to bleed oil during its lifetime. Desirably, a filling material which has an oil separation no greater than 2 percent when centrifuged at relative centrifugal forces of 27,000 g at 25.+-.2.degree. C. for two hours. Because cable drip is related to oil separation, constraints on the sought-after filling material include oil separation, critical-yield stress and viscosity. These constraints are antagonistic to each other. For example, a reduction of oil separation and an increase in cable drip temperature require high viscosity and high yield stress; whereas low viscosity and low yield stress are required to facilitate processing and to reduce optical loss.
Another filling material having a relatively low critical-yield stress is shown in U.S. Pat. No. 5,187,763 that issued on Feb. 16, 1993. The disclosed composition comprises synthetic oil, an inorganic gelling agent (fumed silica), and a bleed inhibitor. Because of the type of gel-forming agents used in the prior art, the network structures formed in the filling materials are not rigid and can be broken and reformed either at the same sites to restore the old structures or at new sites thereby form different structures. The disruptions and reformation of networks are the major reason that these gel-like filling materials display unstable viscosities and cause some difficulties in measuring them. Most critically, such network structures also continue to form resulting in a continuously increasing gel viscosity and/or critical-yield stress to an undesirable level. Obviously, the gel-like filling materials will have lost, over time, most of their desired properties that the prior art had originally intended.
What is sought after and what does not appear to be disclosed in the prior art is an optical fiber cable filling composition of matter which is compatible with a broad range of optical fiber coating materials, which does not bleed, which does not drip from the cable core at specified elevated temperatures, and which has a relatively low critical-yield stress. Importantly, what is also sought after is an optical fiber cable filling composition of matter which has a stable network structure that retains all properties, particularly viscosity, over its lifetime